The present embodiments relate to reducing image artifacts.
Imaging X-ray devices typically have X-ray tubes that serve as radiation sources. Such X-ray emitters supply X-ray radiation with a polychromatic X-ray spectrum (e.g., photons with different energies are emitted). After passing through an object under examination, the attenuated X-ray radiation is detected by an X-ray detector. Photons of higher energy, however, are in general less strongly attenuated than photons of lower energy on passing through material. This effect, in conjunction with the fact that the methods used for image reconstruction (e.g., in tomography) are based on a linear dependency between attenuation coefficients and transirradiated path length, leads to image artifacts that are designated radiation-hardening (“beam hardening”) or “cupping” artifacts. Other artifacts appear in the form of stripes or as dark shadows of strongly attenuating objects or object regions in the reconstructed image.
Beam hardening artifacts occur whenever a polychromatic X-ray spectrum is used for acquiring image data. A reduction in such artifacts is therefore desirable in different technical uses (e.g., on operation of medical or material-examining imaging X-ray devices) in order to improve the image quality.
In order to reduce beam hardening artifacts, different procedures have previously been proposed that may be roughly divided into “hardware”-related and “software”-related modifications, according to approach. An apparatus or “hardware”-related approach would be, for example, to modify the X-ray emitter(s) such that only at least approximately monoenergetic X-ray radiation is emitted, to configure the X-ray detector for energy-discriminating data-acquisition or to use pre-filters. These approaches are either expensive in their realization or reduce the signal-to-noise ratio.
Method-wise modifications, by contrast, may easily be implemented with the aid of correspondingly programmable evaluating units and computer programs. A method for reducing image artifacts caused by beam hardening is described, for example, by M. Kachelrieß et al. in “Empirical cupping correction: A first order raw data pre-correction for cone-beam computed tomography,” Medical Physics 33, 1269-1274 (2006). However, in order to carry out the method, an item of auxiliary information relating to the object to be recorded is to be provided, which is typically acquired through separate calibration measurements with phantoms, in the field of medical imaging, for example, with water phantoms (e.g., with water-filled reference objects).
A further approach relates to the utilization of redundancies that are present in the acquired raw data and are reflected, for example, in consistency conditions. A consistency condition of this type, which makes use, inter alia, of the principles of epipolar geometry, is described by A. Aichert et al. in “Epipolar consistency in transmission imaging,” Transactions on Medical Imaging, vol. 34, No. 11, pages 2205-2219, 2015. A direct application to correction models for artifact reduction is, however, numerically complex.
DE 10 2013 200 329 A1 discloses very generally utilizing consistency conditions in order to correct image artifacts.